This invention relates to a method of making ball-screw racks for use in motor vehicles.
Typically, ball-screw racks are made from a single, solid length of bar stock, and the manufacturing process involves a large number of steps, including the following:
1) Shear the bar and grind to size; 2) rough mill or broach rack teeth in one end portion of the bar; 3) heat-treat the end portion of the bar in which the rack teeth are formed, usually;by conduction hardening; 4) temper the heat-treated end portion of the bar; 5) straighten the bar; 6) rough grind a ball-screw thread groove in the other end portion of the bar; 7) heat-treat the other end portion of the bar, usually by induction hardening; 8) temper the other end portion of the bar; 9) straighten the bar; 10) finish mill the rack teeth; and 11) finish grind the ball-screw thread groove.
This typical manufacturing process is both time-consuming and expensive.
The manufacturing process of this invention involves fewer steps which can be summarized as follows:
1) shear the bar and grind to size; 2) rough mill or broach rack teeth in one end portion of the bar; 3) heat-treat the end portion of the bar in which the rack teeth are formed to one hardness in a first scan and the other end portion (on which a ball-screw thread is already manufactured or is to be manufactured) to a different hardness by induction hardening in a second scan; 4) temper the entire bar by induction; 5) straighten the bar; 6) form a ball-screw thread groove in the other end portion of the bar by whirling; and 7) finish mill the rack teeth.
Whirling, referred to in step 6, is a process in which a geometrically oriented cutter removes chips. Cutting is accomplished by a whirling ring rotating at high speed around a slowly rotating work piece. A whirling ring differs from a milling cutter in that the whirling ring uses cutting tools that face radially inwardly towards the center of the ring, not outwardly as on a milling cutter. Because most of the heat generated by whirling is carried away in the cut chips, whirling is done without a coolant and no subsequent heat treat or tempering is required. Hence the bar needs to be hardened and tempered only once, after milling the rack teeth, preferably by a single shot induction scan of the entire bar.
As indicated above, typical prior methods start with a solid length of bar stock and the end result is a solid rack bar. However, it has been discovered that by starting with tubular bar stock, or initially hollowing out a length of bar stock to make it tubular throughout a portion or all of its length, the finished rack bar is considerably lighter in weight without significant compromise on strength or performance.
It has also been discovered that a rack bar may be made starting with a rack end portion and a separate ball-screw end portion and butt welding the two together end-to-end. The rack end portion may be cut with rack teeth, heat treated, tempered and straightened, and the ball-screw end portion cut with a ball-screw thread groove, heat treated and tempered before the two end portions are resistance butt-welded together. The ball-screw end portion which is usually a few inches long, may, if desired, be cut from bar stock of much greater length, say 10 or 20 feet long. Bar stock with the ball-screw thread groove already formed in it by cold or profile rolling, and from which the ball-screw end portions may be cut, is readily available for purchase by the rack bar manufacturer. Each of the two end portions of the rack bar thus formed may be tubular or hollowed out if desired.
One object of this invention is to provide a manufacturing process for making ball-screw racks having the above features and capabilities.
Another object is to provide a process for making a ball-screw rack which involves fewer steps and less time and expense than processes presently in use. Reduction in multiple heat treat and straightening operations should improve quality of the ball-screw rack (teeth and screw form).